As a semiconductor optical sensor used to detect feeble light, an avalanche photodiode has been developed and used. This diode may be fabricated by using various semiconductor materials such as InP, GaAs, SiGe, and SiC. For example, the avalanche photodiode fabricated by using silicon has a diode structure in which doped layers of p+, n, n−, and n+ or n+, p, p−, and p+ are stacked in parallel to a light receiving surface and operates by applying a voltage slightly lower than a breakdown voltage in a reverse direction to the diode. When a photon is incident in this diode, a pair of electron/hole is generated by photoexcitation. Avalanche multiplication occurs while the pair of electron/hole is accelerated by an electric field, so that the amount of current is amplified to detect weak light with high sensitivity.
Based on such basic structure and operational principle of the avalanche photodiode, a semiconductor photomultiplier operated in a Geiger mode has been developed. The device is similar to an avalanche photodiode, but is to be operated in a voltage range which exceeds the breakdown voltage. According to the operating mode of the device, the number of charge carriers generated by photoexcitation is progressively increased through impact ionization, and a breakdown phenomenon in which charges accumulated in the diode structure flow in a moment occurs. The current-to-photon gain is determined by the overvoltage—the operation voltage subtracted by the device breakdown voltage—, regardless of the number of photons simultaneously incident in a single diode.
In such a Geiger mode photomultiplier type detector, multiple microcells composed of the diode structure connected to each other in parallel constitute one device, and the intensity of current signals simultaneously generated from the microcell array is measured to determine the number of incident photons on the device. Therefore, the dynamic range of the light intensity measurement depends on the area of a single microcell and the number of the microcells to the total area of the device, and a high-density cell pattern is needed in order to improve the dynamic range. Herein, it is noted that when breakdown occurs in one cell, the cell becomes inoperable for some time, and that when a large current flows continuously, the device may be damaged. Therefore, the device needs to be restored from the breakdown state within a short time. To this end, a passive quenching method in which a quench resistor is connected in series to the diode structure of the microcell is generally applied. The quench resistor causes ohmic voltage drop to decrease the voltage applied to the device down to the breakdown voltage when a surge current flows by electrical breakdown, thereby quenching the breakdown state.
When the photon number measurement device is implemented as above through the multiple microcell configurations, the so-called optical crosstalk needs to be suppressed. When the breakdown occurs in one cell by the avalanche phenomenon, multiple photons may be generated during the impact ionization accompanied thereby, and when the photons are transferred to the adjacent cells, the breakdown phenomenon may additionally occur concurrently in the adjacent cells. In order to suppress the optical crosstalk phenomenon, a method of putting a distance between the light receiving units of the cells was primarily adopted in the related art.
In general, multiple microcell semiconductor photomultipliers are configured to maximally widen the actual light receiving area by placing the quench resistors and metal electrodes on the separation space between the cells. However, in the case of high-density cell structure, the ratio of the effective light receiving area to the total device area is significantly reduced due to the influence of the dead area such as the area covered with the quench resistors and the metal electrodes, and the separation area between the cell diodes.
In order to solve the problem, Korean Patent Application Laid-Open No. 10-2009-0129123 disclosed a method of refractively guiding light incident from the top of the device toward the light receiving area by forming a microlens at the upper side of each microcell of the device. However, in this method, there is a limit that the lens is configured by organic materials for which reflow is generally easy, while the refractive indices of these materials are limited to the range of 1.45 to 1.7. When the device fabricated by the above method is used in the air, the refractive index of the lens material is appropriate, but durability of the lens can be problematic. Meanwhile, when the device is used coupled with a scintillator as in a gamma ray detector, optical grease is applied to contact the top of the device to the scintillator. However, since the optical grease also has a refractive index of a similar value (approximately 1.5), the expected light harvesting effect by the lens becomes invalid.
According to the present disclosure, light incident in the dead area of the semiconductor photomultiplier is guided to the light receiving area through reflection and refraction to improve the light receiving efficiency of the device by forming an optical structure on the device.